ABSTRACT: We demonstrate that the silica shell on nanoparticles
formed by a typical St€ober method is inhomogeneous
in nature. The outer layer of the shell is chemically
more robust than the inner layer, which can be selectively
etched by hot water. Methods are developed to “harden”the
soft silica shells. These new understandings are exploited to
develop versatile and template-free approaches for fabricating
sophisticated yolkshell nanostructures.